Natural (autogenic and allogenic) bone tissue is commonly used for bone replacement to correct defects caused by disease or trauma. However, natural bone tissue is not available in sufficient quantities to meet the growing demand. Further, there is a risk of viral infection associated with the use of transplanted tissue. Synthetic materials currently available are limited by inadequate mechanical properties, poor implant-tissue interfacial bonding, or both. Initial implant stability is enhanced if the implant is able to rapidly bond to the surrounding tissue. New orthopaedic synthetic biologically active materials are needed which are readily available and have bonding and mechanical properties comparable to that of natural bone tissue.
The major mineral phase in bone, hydroxyapatite ("HA"), which has the chemical formula: Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2, is able to slowly bond with bone in vivo. By contrast, the inert metals, such as stainless steel and titanium, used in the construction of implanted orthopaedic devices are not generally considered to bond to bone or soft tissue and are generally attached by mechanical means such as pins and screws. In many cases, it would be desirable for the devices to bond to body tissue. That is, material from which the devices were constructed should be biologically active, i.e. "bioactive." Thus, researchers have focused on developing HA coatings for orthopaedic implants which would allow the implants to become bound to body tissue. Unfortunately, the bond which forms between HA and metal implants is weak and subject to fracture. Also, the long term effects of HA coatings relative to uncoated, mechanically bound prostheses are unknown.
The bioactivity index is a measure of the time required for greater than 50% of the interface of a material with bone to become bonded to the bone. The bioactivity index of hydroxyapatite is 3.1. In comparison, the indexes for certain biologically active glasses composed of SiO.sub.2, Na.sub.2 O, CaO and P.sub.2 O.sub.5, "bioactive glasses" as they are known, are significantly higher. For example, for the particular bioactive glass, 45S5 BIOGLASS.RTM., the index is reported to be 12.5. Bioactive glasses when exposed to aqueous solution, e.g., simulated body fluid or water, the outer silica layer becomes hydrated and serves as a nucleation site for precipitation of amorphous calcium phosphate, which becomes crystalline with time. Silicon ions released form the hydrated layer appear to enhance the proliferation of osteoblasts, the cells which build bone. Thus, when immobilized against bone for two weeks 45S5 BIOGLASS.RTM. forms an interfacial bond as strong as the bone itself. [Filho, O. P. LaTorre, G. P. and Hench, L. L. (1996). "Effect of Crystallization on apatite-layer formation bioactive glass 45S5, J. Biom. Mater. Res. 30, 509-514.]
Although bioactive glass has a significant advantage over hydroxyapatite because it is able to rapidly bond to bone and soft tissue, its mechanical properties are insufficient to allow it to be used for load-bearing applications including use as a bonding medium for implants and for extensive bone replacement. For example, researchers have attempted to coat metallic implants with bioactive glass in efforts to impart the surface with the ability to bond with bone and surrounding tissue. However, the metal-glass bond was not strong enough to be practical. Thus, significant improvement of the mechanical properties of bioactive glasses was needed to meet the demands of load bearing applications.
Attempts to produce laminate composites with (a) high strain to failure and (b) a bioactive coating have been disappointing. Development of a bioactive laminate with flexural strength (100 MPa) and strain to failure equal to that of bone (8%) would be of clinical significance.
The use of bioactive ceramics and glass in polymer composites is known in the art (see, for example, U.S. Pat. Nos. 5,017,627 and 5,728,753 to Bonfield et al.). These patents teach the use of a dispersed phase of a bioactive material, either bioactive glass, or hydroxyapatite, in a polyolefinic matrix. These materials are limited in their ultimate tensile strength, and do not possess the fracture toughness necessary to be a fully weight-bearing bone implant. The dispersed phase, whether a particulate or a fiber, can act as a stress-riser, which limits the usful mechanical properties of the material. In addition, they only have a percentage of bioactive material at their surface. It would be advantageous to provide materials with a bioactive surface which increases the bone and soft tissue bonding ability of the material, while gaining significant tensile strength properties and fracture toughness higher than conventional bioactive composite materials.